Your OpenID account is as secure as your OpenID provider wants to make it. That is, with OpenID you can add additional security features like one-time passwords or SMS codes so that you need more than just your password to access your account.

Also consider that a malicious individual would also only need to break into your email account to reset many of your other passwords or take over your account. OpenID potentially improves your security situation by reducing the number of sites that have access to your password.